Measurements while drilling or coring using a wireline drilling machine

ABSTRACT

A drilling system and method for drilling underground boreholes is provided. The drilling system comprises a tool body, a downhole motor, a drill bit mounted on the body to be driven by the motor, a mechanism for driving the tool body axially along the borehole so as to apply weight on the drill bit when drilling, and an electrical cable extending from the surface to the tool body to provide power for the drilling motor. The tool body includes a conduit connected to the drill bit such that drilled material passes from the drill bit through the conduit, and sensors are provided in the conduit for measuring properties of the drilled material.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on and claims priority to GB Application No. 0722444.7, filed Nov. 15, 2007; and International Patent Application No. PCT/EP2008/009606, filed Nov. 6, 2008. The entire contents of each are herein incorporated by reference.

TECHNICAL FIELD

This invention relates to techniques for making measurements during drilling or coring using a wireline drilling machine. In particular, the invention relates to the use of an instrumented drilling machine for making such measurements.

BACKGROUND ART

Measurements while coring has been previously proposed for conventional drilling applications. U.S. Pat. No. 6,788,066 discloses such a system in which sensors for making electromagnetic propagation measurements are provided on the inside of downhole cylindrical enclosure for measuring properties of drilled cores such as resistivity/conductivity of pore fluid, dielectric constant of rock matrix and water-filled porosity.

Recent proposals for drilling underground boreholes have included the use of a wireline drilling machine. Such machines differ from conventional jointed pipe or coiled tubing drilling in that there is no mechanical link to the surface to provide weight on bit. The drilling machine is provided with power which it must convert into weight on bit itself, for example by use of a downhole tractor type device.

DISCLOSURE OF THE INVENTION

A first aspect of this invention provides a drilling system for drilling underground boreholes, comprising:

a tool body;

a downhole motor;

a drill bit mounted on the body to be driven by the motor;

a mechanism for driving the tool body axially along the borehole so as to apply weight on the drill bit when drilling; and

an electrical cable extending from the surface to the tool body to provide power for the drilling motor; wherein the tool body includes a conduit connected to the drill bit such that drilled material passes from the drill bit through the conduit, and sensors are provided in the conduit for measuring properties of the drilled material.

The system preferably also comprises a pumping system which is operable to pump drilling fluid through the bit and conduit.

The bit can be provided with a central aperture so as to drill cores. In this case, the sensors measure the properties of the core. In another, the bit can be configured to create cuttings. In this case, the sensors measure the properties of drilled cuttings and any drilling fluids passing through the conduit.

It is particularly preferred that the sensors are arranged around the periphery of the conduit. This can allow tomographic measurements across the whole section of the conduit.

The sensors can be arranged to measure resistivity, electromagnetic propagation, acoustic response, natural radioactivity, gamma ray, neutron and x-ray radiation, and nuclear magnetic resonance. These measurements can be used to obtain rock and pore fluid petrophysical and chemical properties.

A second aspect of the invention comprises a method of drilling a borehole using a system according to the first aspect of the invention, the method comprising:

operating the drilling system so as to drill through the underground formation;

passing drilled material through the conduit; and

measuring properties of the drilled material using the sensors.

The measured properties can be used to provide detection of events such as gas kicks and abnormal cuttings size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a drilling tool according to an embodiment of the invention.

MODE(S) FOR CARRYING OUT THE INVENTION

Referring now to FIG. 1, the drilling tool shown therein comprises a tool body 10 that is lowered into a borehole 12 on a cable 14. The borehole can be, for example, a lateral borehole drilled from a main borehole, such as might be used to enhance production or reach bypassed production zones. The cable 14 provides electrical power and, optionally, data for controlling the tool and retrieving operational and measurement data. In certain cases, an umbilical including tubular conduits as well as the electrical cable may be used. A drilling motor 16 is provided at the end of the tool body 10. Power for the motor is provided by the cable 14. Alternatively, a motor driven by fluid flow can be used but this requires some means of pumping fluid into the motor. A drill bit 18 is mounted on the tool body 10 so as to be driveable by the motor 16. A tractor mechanism 20 is provided at the other end of the tool body 10. This mechanism can be similar to that described in EP04292251.8 and PCT/EP04/01167 and can operate to advance the tool body along the borehole.

A central flow passage 22 extends through the bit 18 and body 10 to exit at the rear of the tool. An array of sensors 24 is provided along and around the flow passage 22. A particularly preferred form of sensor is an electrode for performing electrical impedance tomography (as will be discussed in more detail below) although other sensors such as optical sensor can also be used. Other sensors (not shown) are also provided for drilling measurements (TOR, WOB, RPM, ROP, DOC) and direction and inclination measurements in the usual manner.

In use, the tool is lowered into the borehole and the tractor 20 operated to advance the tool along the borehole 12. The motor 16 is operated to rotate the bit which drills ahead when contacting the formation at the end of the borehole. Drilling fluid 26 is pumped around the outside of the tool so as to flow through the bit and along the passage 22. In one configuration, the tool is operated so that the bit creates cuttings of the drilled material which are carried with the flow of drilling fluid up inside the passage 22 and past the sensors 24. As the drilling fluids and cuttings pass the array of sensors, measurements are made along the array and around the passage so as to provide a tomographic measurement of the flow. These measurements can be used as a raw image or processed to identify particular events. Examples of the events that can be identified by such measurements include gas kicks (gas influx from the formation) or the presence of abnormal cuttings (which can potentially damage the downhole equipment or indicate drilling problems).

Other sensors that can be used include ultrasonic sensors that can be used for imaging purposes. Chemical sensors can also be used to detect compositional changes in the drilling fluid that can be indicative of drilling through certain types of formation or H₂S risk detection. A combination of a variety of sensors measurements and a data processing apparatus may be used to resolve ambiguous events in the flow. For example, while an increased impedance may be an ambiguous signal of either a gas bubble or of a sizeable hard rock cutting, this ambiguity is resolved for example by the simultaneous signal of pressure sensors along the path of the flow , or by images from ultrasonic sensors.

In another configuration, the bit is arranged to drill a solid core from the formation, which passes into the passage. In this case, the sensors measure the formation properties directly. This has the advantage that it avoids the presence of measurement artefacts that arise due to core decompression and core recovery that can arise when making such measurements at the surface after retrieving the core from downhole. It will be understood that a combination of several arrays of sensors may likewise provide a better petrophysical evaluation of the core material.

A number of other benefits can also be obtained. The .presence of the wireline cable allows data recovery at high data rates when compared to previous coring while drilling applications. Also , that data is acquired very close to the depth at which rock is being drilled, and close to instantaneously transmitted at surface, allowing a fast and precise reaction to equipment and human safety hazards in the drilling operation such as: abnormal drill cuttings, gas kicks, sour (hydrogen sulfide) gas influx.

Another drilling risk that can be monitored by such apparatus is the risk (under some specific mud, temperature and pressure conditions) of formation of solid methane hydrates in the mud, that would lead to a loss of the mud rheological properties, plugging of the flow lines and further destabilization of the drilling and pressure control system.

Traditional mud logging operations rely on measurement on the mud and cuttings materials transported by the mud flow at surface , introducing both a sizeable time delay and a significant error on the origin depth of these materials and related events: the use of such wireline drilling machine with embedded mud and cutting measurements minimizes both delay and depth error. The use of a wireline drilling machine makes the recovery of cores from extended laterals possible, something that has not been possible to date using conventional coring technology due to its limited reach away from the main well.

Further changes within the scope of the invention will be apparent. 

1. A drilling system for drilling underground boreholes, comprising: a tool body; a downhole motor; a drill bit mounted on the tool body to be driven by the motor; a mechanism for driving the tool body axially along the borehole so as to apply weight on the drill bit when drilling; and an electrical cable extending from the surface to the tool body to provide power for the drilling motor; wherein the tool body includes a conduit connected to the drill bit such that drilled material passes from the drill bit through the conduit, and sensors are provided in the conduit for measuring properties of the drilled material.
 2. The system as claimed in claim 1, further comprising a pumping system which is operable to pump drilling fluid through the drill bit and conduit.
 3. The system as claimed in claim 1, wherein the drill bit is provided with a central aperture so as to drill cores.
 4. The system as claimed in claim 3, wherein the sensors measure the properties of the core.
 5. The system as claimed in claim 1, wherein the drill bit is configured to create drilled cuttings.
 6. The system as claimed in claim 5, wherein the sensors measure the properties of drilled cuttings and any drilling fluids passing through the conduit.
 7. The system as claimed in claim 1, wherein the sensors are arranged around the periphery of the conduit.
 8. The system as claimed in claim 7, wherein the sensors are configured to provide tomographic measurements across the whole section of the conduit.
 9. The system as claimed in claim 1, wherein the sensors measure resistivity, electromagnetic propagation, acoustic response, natural radioactivity, gamma ray, neutron and x-ray radiation, and/or nuclear magnetic resonance.
 10. The system as claimed in claim
 9. wherein the measurements are used to obtain rock and pore fluid petrophysical and chemical properties.
 11. A method of drilling a borehole through an underground formation, comprising the steps of: providing a drilling system comprising: a tool body; a downhole motor; a drill bit mounted on the tool body to be driven by the motor; a mechanism for driving the tool body axially along the borehole so as to apply weight on the drill bit when drilling; and an electrical cable extending from the surface to the tool body to provide power for the drilling motor; wherein the tool body includes a conduit connected to the drill bit such that drilled material passes from the drill bit through the conduit, and sensors are provided in the conduit for measuring properties of the drilled material operating the drilling system so as to drill through the underground formation; passing drilled material through the conduit; and measuring properties of the drilled material using the sensors.
 12. The method as claimed in claim 11, wherein measured properties are used to provide detection of gas kicks and abnormal cuttings size. 